High permittivity materials are characterized by a dielectric constant, R, of 100 or more, well above that of conventional dielectrics, such as silicon dioxide (R=4). A large class of materials with high permittivity is known as the titanates, and the most widely used material in this class is BaTiO.sub.3, or barium titanate. The general class of materials has an ATiO.sub.3 chemistry, where "A" is typically Ba, but may also be a similar element from the periodic table. Variations on this chemistry have been developed which have minor or part substitutions for the Ba or the Ti, while retaining the same overall structure. An example of a partial Ba substitution is Barium Strontium Titanate (BST), in which approximately 10-15% of the Ba is replaced with Sr. An example of partial Ti substitution is Barium Zirconate Titanate (BZT), in which about 15% of the Ti is replaced with Zr. These substitutions may result in enhancement of either the dielectric constant or the polarizability of the material.
The titanates all have a perovskite crystalline structure, which is a widely known variation of a cubic lattice. However, this particular crystal structure is not known to form at low temperatures, and typically does not form until the material has been raised to at least 650.degree. C. This means that virtually all fabrication techniques for titanates require an annealing step at 650.degree. C. or higher to form the desired perovskite structure and the high dielectric constant.
Titanate films can be deposited by various conventional techniques, such as chemical vapor deposition (CVD), physical vapor deposition (PVD), sol-gel techniques, or pulsed-laser deposition (PLD), and then this titanate film is annealed to form the desired perovskite structure and also to ensure adequate oxygen levels within the deposited high permittivity material. This application relates to the physical sputtering techniques and the annealing process to form high permittivity films of materials such as the titanates.
First, sputter-deposition techniques fall into two classes; diode sputtering and ion beam sputtering. This application is related primarily to diode sputtering, which in practical usage constitutes either rf diode or magnetron sputter deposition. Ion beam sputtering is characterized by fairly low deposition rates and a moderate degree of complexity for the ion beam system, and hence has fairly little application in large area, high rate, or manufacturing scale operations.
Diode sputtering systems include a cathode and an anode in a vacuum system. A plasma is formed between the electrodes following the application of a suitably high dc or rf potential between the electrodes. Ions from the plasma bombard the cathode causing physical sputtering, which results in film deposition on nearby surfaces. This technology is known widely as sputter deposition. The addition of specialized magnetic fields to the cathode can enhance the sputtering and deposition rates, and cathodes of this type are widely known as magnetrons. Diode sputtering is widely used for the deposition of metallic films, and can also be used for the reactive deposition of compound films when sputtering is performed in the presence of a reactive gas species.
A fundamental problem has been observed when sputtering materials in the titanate class. This problem has also been observed when sputtering other perovskite materials, such as the high temperature superconductors (yttrium-barium-copper-oxides and related materials). During the sputtering process from a composite target, negative oxygen ions are formed at the cathode surface. This is thought to be due to the high electron affinity for oxygen and also the presence of species, such as Barium, which are prone to giving up an electron. This effect is widely observed with conventional PVD composite targets. The composite targets are formed by hot-pressing mixed powders of the appropriate oxides into disks. For example, a barium-titanate sputter-target would be formed by making a mixture of barium oxide and titanium oxide articles, and hot-pressing them into a disk shape which is then bonded to a target plate.
Once the oxygen negative ion is formed at the cathode surface, it is accelerated into the plasma by means of the cathode dark-space or sheath. This is a similar process to the acceleration of secondary electrons into the plasma from the cathode. As the oxygen negative ion crosses the cathode sheath, it attains a kinetic energy equal to the cathode potential and then enters the plasma. Its trajectory is normal to the cathode surface. Once in the plasma, the extra electron on the oxygen negative ion is stripped by the background plasma, and the oxygen ion becomes a neutral atom. However, it is still moving with high energy away from the cathode.
Most deposition systems place the sample-to-be deposited-on at a location opposite the cathode with a cathode-to-sample distance of perhaps 5-25 cm. The energetic oxygen neutral atom crosses through the plasma and can bombard the sample surface with considerable energy, leading to the sputtering (or `re-sputtering`) of the deposited film on the sample surface. This resputtering process has several drawbacks. First, it results in a significant reduction in the net deposition rate at the sample position as the energetic neutral oxygen atoms bombard the film surface. Second, since the sputtering rate varies depending on the material being sputtered, the energetic neutral resputtering of the film surface may preferentially remove atoms with high sputter yields from the depositing film, altering its chemical stoichiometry. The result of these two processes is to significantly degrade the sputter deposition rate and to deposit a film which has a stoichiometry which is different from the cathode being sputtered.
The effect of negative ion-energetic neutral re-sputtering is also exaggerated by the presence of additional oxygen atoms in the plasma discharge chamber. Often when materials such as the titanates are sputtered, additional oxygen must be added to the deposition chamber to help deposit the films in a more completely oxidized state. The added oxygen from the gas phase can lead to additional negative ion formation at the cathode and increased bombardment of the deposited film by negative ions. Several reports have been published which show significant etching due to negative ion effects when sputtering titanate or high temperature perovskite superconductors.
There are only two known solutions to this problem, prior to the current application. The first utilizes very high gas pressures during sputtering. Typical pressures used are many hundreds of milli Torr, where typically sputtering would normally operate at a few to 30 or so milli Torr. The high gas pressure slows the energetic negative ions/neutrals by gas phase collisions and this reduces the problem of re-sputtering. The deposition rate, however, is strongly reduced by the high pressure, and this solution is then inappropriate for manufacturing applications.
Further, all deposition technologies for high permittivity materials utilize a high temperature annealing step in oxygen. The standard technique is to anneal the films for a period of a minute or so up to several hours in a background of oxygen, air or some other oxidizing ambience. Generally, the dielectric constant achieved is related to the temperature of the anneal. For example, an anneal of BZT to a temperature of 650.degree. C. has been observed to lead to a dielectric constant of up to 300-330. The same film, when annealed to 900.degree. C. leads to a dielectric constant of 550-600.
This oxidizing, high temperature anneal may cause significant problems with other materials which may be part of the sample substrate because these materials may also be oxidized. It may also cause structural problems, as the physical dimensions of the material may change as it is oxidized, causing expansion or contraction of underlying features on the sample.
To prevent oxidation, oxygen diffusion barriers have been used to protect underlying features. Typical diffusion barriers might include TiN, TaN or TaSiN, each of which may have slight stoichiometric permutations. However, all known diffusion barriers have been found to fail in the presence of oxygen as the temperature exceeds approximately 700.degree. C.
The failure of the diffusion barrier severely limits the optimization of the dielectric constant. It may also prove limiting to the optimization or minimization of the losses in the dielectric, which have been observed to be reduced by annealing in oxygen.